Chemical vapor deposition, or CVD is a commonly used technique for applying material films or layers to a substrate in the formation of integrated circuits. CVD comprises introducing various reactant gases into a deposition chamber housing a substrate. The reactant gases mix proximate the substrate and chemically react at the surface of the substrate. One or more reactant products from the chemical reactions deposit upon the substrate surface and form a film.
One form of CVD is plasma-enhanced chemical vapor deposition or PECVD. In PECVD, one or more of the reactant gases are excited into a plasma such as by being exposed to RF or microwave electrical energy. The plasma includes various activated particles of the gas or gases. The excited plasma is mixed with other reactant gases, and the plasma supplies energy to the chemical reaction between the various gases to deposit a film on a substrate.
As may be appreciated, the flow of the reactant gases to the substrate surface and to the plasma is important to ensure proper deposition of films in both CVD and PECVD. Preferably, the flow of the plasma gases to the excited plasma in PECVD, in addition to the flow of the other reactant gases to the substrate surface are uniform to promote uniform deposition of the desired film.
In some CVD techniques, the reactant gases are introduced at predetermined flow rates and evacuated at similar rates to ensure that the reactants are propelled in sufficient densities to react and form the desired film. Generally, the reactant gases are introduced above a substrate, such as by a gas ring or halo, and travel downwardly to the substrate at predetermined flow rates. Upon reaching the substrate, the gases mix and react to form a film and any remaining gases are exhausted such as by a vacuum system. In such CVD techniques, there is usually a stagnant layer between the gas flow of the mixed reactant gases and the substrate surface where very small densities of reactants are present. Such a stagnant layer is referred to as a boundary layer. When the boundary layer is large, an undesirable amount of the reactant gases may bypass the substrate and be exhausted from the reaction chamber without reacting. This is wasteful, and therefore costly. It is preferable to have a boundary layer as thin and flat as possible so that a useful density of the gas reactants used in the chemical reaction are available at the substrate surface and do not bypass the substrate to be exhausted, unreacted, out of the chamber.
One way of achieving a thin boundary layer at the substrate is to introduce the reactant gases under matched flow conditions. Matched flow of reactant gases is achieved when the outward volume of gas flowing parallel to and over the flat surface of the substrate is approximately the same as the input volume of gas flowing generally downward and perpendicular to the substrate surface. With low gas flow rates, matched flow can usually be readily achieved; however, with higher gas flow rates, the reactant gases at the substrate do not flow outwardly over the surface of the substrate rapidly enough, and hence, turbulence and backflow of the downward gas flow results.
One alternative for reducing such backflow and turbulence at increased input gas flow rates is to rotate the substrate on a rotating susceptor. An example of a suitable rotating susceptor is utilized within the Rotating Disk Reactor available from Materials Research Corporation (MRC) of Phoenix, Ariz. A rotating susceptor spins the substrate and creates a downward and outward pumping action which draws the reactant gases to the surface of the substrate and outwardly over the surface. The pumping action creates a more rapid outward flow of the gases over the substrate to allow a higher downward gas flow rate without backflow and turbulence. Preferably, the wafer is rotated at a speed which achieves matched flow, i.e., where the downward flow rate is equal to the outward flow rate. Matched reactant gas flow using a rotating susceptor ensures that a suitably thin boundary layer of reactant gas is present for uniform deposition of a film.
While the use of a rotating susceptor allows greater gas input flow rates, it has generally been found that the velocity profile of the reactant gases pumped by the susceptor should be fully developed before the gases reach the rotating substrate surface in order to obtain a uniform flow over the substrate and thus uniform deposition on the substrate. That is, the velocity of the incoming gas flow as measured across the flow path should reach a steady state. To achieve a steady state flow using currently available CVD apparatuses at useful deposition pressures (e.g. from 1 to 100 Torr), it has been necessary to space the gas ring and gas-dispersing showerhead or other gas input device around four (4) inches or more from the surface of the rotating substrate. While enhancing the steady state flow of the gas at the substrate, such a large spacing is not without its drawbacks.
One significant drawback is that the incoming reactant gases disperse when traveling such a large distance between their point of introduction and the rotating substrate. With such dispersion, an appreciable volume of the reactant gases bypass the substrate around the substrate edges and exit the reaction chamber without reacting at the substrate surface. For example, FIG. 1 shows various streamlines 5 of a downward and outward reactant gas flow within a CVD reaction space 7 which houses a substrate 8 which rotates on a rotating susceptor 6. The streamlines 5 are from gas rings and a gas-dispersing showerhead (not shown) spaced approximately 4 inches or more above susceptor 6 and substrate 8. The streamlines 5 illustrate what occurs when such a large spacing is used between the gas-dispensing rings and showerhead and the rotating substrate 8. As may be seen, the average size of the boundary layer, indicated generally by reference numeral 10, is fairly significant and a substantial amount of the reactant gases 5 bypass rotating substrate 8 and pass around the baffle 11 to be exhausted out of the reaction space 7 by an appropriate exhaust system (not shown). The significant bypassing of the gases 5 lowers the deposition rate because there is a reduced density of reactants available at the substrate surface 12 for the surface reaction. Furthermore, the wide boundary layer of reactant gases 5 at the substrate surface 11 affects the uniformity of the film deposited on substrate 8. Still further, the wasted, unreacted gases which are exhausted make the overall deposition technique inefficient and costly.
Another drawback to the large spacing between the gas dispensing and dispersing structures and the rotating substrate is the inability to ignite a sufficiently dense plasma proximate the substrate. Specifically, in PECVD techniques, it is desirable to generate a reactant gas plasma close to the substrate so that a sufficient density of activated plasma particles are present to provide energy to the surface reaction. Particularly, a concentrated plasma is necessary for low-temperature PECVD of titanium-containing films as disclosed in the U. S. patent application entitled "Method And Apparatus For Producing Thin Films By Low Temperature Plasma-Enhanced Chemical Vapor Deposition Using A Rotating Susceptor Reactor" which is being filed on the same day herewith. However, igniting a suitably dense plasma proximate the rotating substrate while maintaining a steady state gas flow to the plasma has not been satisfactorily achieved with current apparatuses utilizing gas rings and showerheads spaced four (4) or more inches from the rotating substrate.
Therefore, it is an objective of the present invention to disperse the reactant gases at substrate surface such that there is a small boundary layer and sufficient densities of the gases at the substrate surface while maintaining a steady state gas flow to the substrate. Further, it is an objective to produce a dense plasma at the substrate surface such that the plasma is sufficiently concentrated at the substrate surface to yield deposition of a PECVD film.